This invention relates to optical amplifiers and in particular to fibre Raman amplifiers.
Fibre Raman amplifiers generally employ an optical pump signal which is continuous wave and of an appropriate wavelength, one or more Stokes shift away from the wavelength of the signal to be amplified. For example a 1.46 micron pump for amplification of a 1.55 micron signal. An input data stream signal and the pump signal interact in a conventional transmission fibre and gain is achieved in the data stream signal by the Raman effect. Fibre Raman amplifiers have not received much attention since the introduction of erbium fibre amplifiers since the latter are much more power efficient. The same order of gain can be achieved for at least an order of magnitude less mean power. Raman fibre amplifiers require a pump source giving typically 200 mW continuous light whereas erbium fibre amplifiers require about 20 mW for the same gain.